Facies patterns and stratal geometries: clues to the nature of the platform margin during the Paleocene, northeast Sirte Basin, Libya

Abstract

The Sirte Basin of Libya is a Mesozoic—Tertiary rift basin comprising a series of horsts and grabens, which were formed as a result of collapse of a N—S trending arch in Late Cretaceous time. The Paleocene section is made up of two shallowing-upward cycles, where each cycle is under- and overlain by deeper, pelagic facies. A gradual shallowing trend from basin to shallow platform is seen in both bio- and lithofacies. These are interpreted as representing prograding ramps, with no evidence of a significant break in slope between the basin and the adjacent platform. The fact that most facies are dominated by micrites suggests that deposition was in a low-energy setting, possibly on a leeward margin. These ramps were initially foram-dominated platforms, setting the style for later Tertiary ramps, large forams filling the role of inner-mid ramp carbonate producers after the demise of the rudist—coral assemblage at the end of the Cretaceous.Based on local and regional information, a number of episodes of relative sea-level change can be recognized. The two shallowing-upward ramp cycles are interpreted as representing two highstand system tracts. The precipitation of evaporites in the lower cycle appears to be associated with the earlier highstand system tract, culminating during a sea-level lowstand. A drop in sea level during the Late Paleocene is also invoked to explain the interpreted influx of meteoric waters following the later highstand. This event resulted in the creation of good mouldic and vuggy porosities, thus making these porous, inner-mid ramp facies potential targets for hydrocarbon exploration. On a basin-wide scale, this event is also very important because all the Paleocene reservoirs, at least in the eastern Sirte Basin, seem to be related to this possible subaerial exposure event.